The invention relates to a circuit arrangement for the field deflection in a picture display device, comprising a sawtooth generator for generating a sawtooth-shaped signal having a trace and a retrace, and a field synchronizing circuit for generaing a local field signal and for applying this signal to the sawtooth generator for each time initiating the retrace of the generated sawtooth-shaped signal, and for synchronizing the locally generated field signal with an incoming field synchronizing signal.
A circuit arrangement of this type is fairly generally known, for example, from U.S. Pat. No. 4,383,273. The sawtooth-shaped signal obtained is applied to a field deflection amplifier applying a current to a field deflection coil for vertically deflecting one or more electron beams generated in a picture display tube. The field deflection current is also sawtooth-shaped but the duration of its retrace does not need to be the same as that of the retrace of the signal generated by the sawtooth generator, because the variation of the said current also depends on the inductance of the deflection coil. In the known circuit arrangements the sawtooth generator is at will designed to obtain a signal of a well defined shape, that is to say, its trace and retrace have given durations.
The invention is based on the recognition that a variation of the shape of the sawtooth-shaped signal may be desirable to increase the flexibility of the circuit arrangement and to this end the circuit arrangement according to the invention is characterized in that it is provided with a switch operated by a control stage for switching over the duration of the retrace period between a first and a second value, the second duration being shorter than the first.
Since according to the invention the duration of the retrace period is switchable between two values, it will be evident that the trace period may also have two durations. In the nominal state of the field synchronizing circuit in which the incoming signal is a standard signal, that is to say, in accordance with a standard at which the repetition frequency of the field synchronizing pulses has a prescribed constant value and in which the locally generated field signal is in synchronism with the incoming signal, the retrace of the sawtooth shape at a constant field frequency is in fact initiated at fixed instants between which the intervals, the field periods, have a prescribed constant duration. In spite of the measure according to the invention the instants of commencement of the field periods remain temporarily fixed, that is to say, they are not shifted so that other signals derived from the generated sawtooth such as, for example, the field blanking pulses are also fixed, whilst the circuit arrangement can be used for several purposes.
In one embodiment the circuit arrangement according to the invention is characterized in that a pulse shaper for deriving a signal from the local field signal for the supply thereof, to the sawtooth generator is connected to the field synchronizing circuit, which signal has substantially the same duration as the retrace of the sawtooth-shaped signal generated by the sawtooth generator, the pulse shaper including a switch for shortening or lengthening the duration of the said signal.
In this case, the sawtooth generator comprising a capacitor and means for successively charging the capacitor during the trace period and discharging it during the retrace period, the circuit arrangement may be characterized in that in the state of the switch in the said pulse shaper in which the retrace period has the first duration, the signal generated by the pulse shaper temporarily interrupts the discharging of the capacitor.
In a first use the circuit arrangement according to the invention is characterized in that for eliminating the interlacing of a picture to be displayed on a picture display tube upon reception of a video signal with interlacing the control signal applied by the control stage to the switch has the picture frequency for obtaining a sawtooth-shaped signal in which the retrace period alternately has the first and the second duration and in which the difference between the first and the second duration is equal to half the line period in the incoming video signal.
Circuit arrangements for eliminating the interlacing upon reception of interlaced video signals, for example, when the picture to be displayed consists of digitally generated picture and/or text information signals are known per se. For example, in European patent application No. 39,554 the interlacing is eliminated in that the field synchronizing signal is alternately delayed by half a line period and not delayed, respectively. Consequently, the instant of commencement of the period of a sawtooth-shaped signal is alternately shifted or not shifted, which produces a picture frequency variation of this instant and may cause difficulties in generating other field signals, for example, the field blanking pulses. On the other hand the measure proposed here only produces a very small amplitude variation of the sawtooth-shaped signal which is little troublesome and can be eliminated in a simple manner.
A circuit arrangement of the type described above may be characterized in that a memory circuit comprising a memory element is connected to the sawtooth generator, as well as a source for applying information to the memory element and a source for removing information from this element, which circuit also comprises a control loop for controlling the quantity of information stored in the memory element, which information has a predetermined substantially constant value when the interval between incoming field synchronizing pulses has a predetermined duration and a value deviating therefrom in the opposite case.
In a second use of the switch-over according to the invention this memory circuit may be used so that an improvement of the field synchronizing is obtained. To this end the circuit arrangement is characterized in that the memory circuit comprises a comparison stage coupled to a counter for comparing the difference between information derived from the peak of the sawtooth-shaped signal and the information stored in the memory element with a reference for each time initiating the retrace and for each time applying a signal to the control stage for changing over the switch to the state in which the retrace period has the second duration, the interval between the instants of commencement of the retrace periods being equal to the said predetermined duration, and this in the absence of a plurality of successive incoming field synchronizing pulses, which plurality is not higher than a predetermined value.